builtin/show-ref: treat directory as non-existing in --exists

9080a7f (builtin/show-ref: add new mode to check for reference
existence, 2023-10-31) added the option --exists to git-show-ref(1).

When you use this option against a ref that doesn't exist, but it is
a parent directory of an existing ref, you get the following error:

    $ git show-ref --exists refs/heads
    error: failed to look up reference: Is a directory

when the ref-files backend is in use.  To be more clear to user,
hide the error about having found a directory.  What matters to the
user is that the named ref does not exist.  Instead, print the same
error as when the ref was not found:

    error: reference does not exist

Signed-off-by: Toon Claes <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: To1ne

builtin/show-ref.c

@@ -239,7 +239,7 @@ static int cmd_show_ref__exists(const char **refs)
 	if (refs_read_raw_ref(get_main_ref_store(the_repository), ref,
 			      &unused_oid, &unused_referent, &unused_type,
 			      &failure_errno)) {
-		if (failure_errno == ENOENT) {
+		if (failure_errno == ENOENT || failure_errno == EISDIR) {
 			error(_("reference does not exist"));
 			ret = 2;
 		} else {

t/t1403-show-ref.sh

@@ -260,9 +260,9 @@ test_expect_success '--exists with non-commit object' '
 
 test_expect_success '--exists with directory fails with generic error' '
 	cat >expect <<-EOF &&
-	error: failed to look up reference: Is a directory
+	error: reference does not exist
 	EOF
-	test_expect_code 1 git show-ref --exists refs/heads 2>err &&
+	test_expect_code 2 git show-ref --exists refs/heads 2>err &&
 	test_cmp expect err
 '